Electronic products such as personal computers, televisions, and Digital Video Disc (DVD) players are widely used in modern daily life. In general, a power supply circuit that outputs a steady voltage and has low power consumption is required in these kinds of electronic products.
FIG. 4 is a schematic diagram of a conventional power supply circuit. The power supply circuit 10 mainly includes a full bridge rectifier circuit (not labeled), a transformer 11, a communicating and filter circuit 12, a sampling circuit 13, an optical coupler 14, a pulse width modulation circuit 15, and a transistor 16.
The sampling circuit 13 includes a first resistor 131 and a second resistor 132. One terminal of the first resistor 131 is coupled to an output terminal 103 of the power supply circuit 10, and the other terminal of the first resistor 131 is coupled to an anode of a diode 17 via the second resistor 132. A cathode of the diode 17 is grounded via a capacitor 19. A third resistor 18 is coupled between the output terminal 103 of the power supply circuit 10 and the cathode of the diode 17. The resistances of the first, second, and third resistors 131, 132, 18 may be 100 ohm, 1000 ohm, and 10 kiloohm, respectively.
In use of the power supply circuit 10, an alternating current (AC) voltage is applied between a first and a second input terminals 101a, 101b of the power supply circuit 10. Then the AC voltage is converted to a direct current (DC) voltage via the full bridge rectifier circuit, the transformer 11, and the communicating and filter circuit 12. The DC voltage is then fed back to the optical coupler 14 via a voltage that is applied to the second resistor 132 of the sampling circuit 13.
A bias voltage is applied to a third input terminal 102 of the power supply circuit 10 to enable the optical coupler 14 to function. Then the feedback voltage is applied to the pulse width modulation circuit 15. According to the feedback voltage, the pulse width modulation circuit 15 adjusts a duty cycle of voltage applied to a gate electrode of the transistor 16. For example, the feedback voltage decreases along with a decreasing of the output voltage of the power supply circuit 10. Then the pulse width modulation circuit 15 increases a duty cycle of voltage applied to the transistor 16. Thereby, the gating time of the transistor 16 is increased so as to increase the output voltage of the power supply circuit 10. In an opposite example, when the output voltage of the power supply circuit 10 increases, the pulse width modulation circuit 15 decreases the gating time of the transistor 16 so as to decrease the output voltage of the power supply circuit 10.
The sampling circuit 13 is configured by including the first and the second resistors 131, 132, therefore there is a current flowing through the sampling circuit 13. This increases the power consumption of the power supply circuit 10. For example, if the resistances of the first and second resistors 131, 132 are 100 ohm and 1000 ohm, the power consumption of the sampling circuit 13 is about 5.3 milliwatt (mW).
Furthermore, when a load 104 is coupled to the power supply circuit 10, the load 104 is connected in parallel with the third resistor 18 and the sampling circuit 13, as shown in FIG. 5. Typically, a resistance of the load 104 varies in a large range. Therefore, the resistance of the parallel circuit configured by the load 104, the third resistor 18, and the sampling circuit 13 varies according to the resistance of the load 104. Because the resistance of the third resistor 18 is much larger than the total resistance of the first and second resistors 131, 132, the resistance of the third resistor 18 can be ignored when considering the resistance of the parallel circuit. Therefore, the resistance of the parallel circuit ranges from 0 ohm to 1100 ohm. That is, voltages applied to the sampling circuit 13 and a power consumption of the second resistor 132 each vary in a large range. As a result, the feedback voltage from the second resistor 132 is liable to be inaccurate. The inaccurate feedback voltage is amplified by the pulse width modulation circuit 15 to adjust the gating time of the transistor 16. Therefore the power supply circuit 10 may output voltage unstably.
What is needed, therefore, is a power supply circuit that can overcome the above-described deficiencies.